Ductile iron pipe
[5][6] Studies of ductile iron pipe's environmental impact have differing findings regarding emissions and energy consumed.
However, it is the external diameter of the pipe that is kept constant between changes in wall thickness, in order to maintain compatibility in joints and fittings.
Australia adopted at an early point the imperial British cast iron pipe standard BS 78, and when this was retired on British adoption of ISO 2531, rather than similarly harmonizing with Europe, Australia opted for a "soft" conversion from imperial units to metric, published as AS/NSZ 2280, with the physical outer diameters remaining unchanged, allowing continuity of manufacture and backwards compatibility.
Individual lengths of ductile iron pipe are joined either by flanges, couplings, or some form of spigot and socket arrangement.
Flanges are designed to a large number of specifications that differ because of dimensional variations in pipes sizes and pressure requirements, and because of independent standards development.
Due to the rigid nature of the joint and the risk of excessive bending moment being imposed, it is advised that flanged pipework not be buried.
The corollary is that unrestrained spigot and socket joints transmit essentially no compression or tension along the axis of the pipe and little shear.
Any bends, tees or valves therefore require either a restrained joint or, more commonly, thrust blocks, which transmit the forces as compression into the surrounding soil.
[10] Over the last 100 years, the average thickness of iron pipes has decreased due to increased metal strength,[12] through metallurgical advancements as well as improved casting technique.
[6] Based on a 2005 meta analysis of 1,379 pipe specimens, loose polyethylene sleeving was found to be highly effective at mitigating corrosion.
In instances of more aggressive soils, polyethylene sleeving is installed over the zinc coated pipe to provide added protection.
[10][18] Ductile iron pipe is highly resistant to internal corrosion in potable water and less aggressive forms of sewage because of the linings applied during manufacturing.
[19] The first cement lined iron pipe was installed in Charleston, South Carolina, in 1922 and provided 100 years of free-flowing service until the mains were replaced during a routine upgrade to the city's infrastructure in 2022.
Polyurethane is an option offered as an internal lining for ductile iron pipes in lieu of cement mortar.
In Europe, standards recommend a more sophisticated system of directly bonded zinc coatings overlaid by a finishing layer be used in conjunction with polyethylene sleeving.
LPS comprises a loose sleeve of polyethylene that completely wraps the pipe, including the bells of any joints.
By providing an impermeable barrier to ground water, the sleeve also inhibits the diffusion of oxygen to the ductile iron surface and limits the availability of electrolytes that would accelerate corrosion.
[20] Water present beneath the sleeve and in contact with the pipe surface is rapidly deoxygenated and depleted of nutrients and forms a stable environment in which limited further corrosion occurs.
An improperly installed sleeve that continues to allow the free flow of ground water is not effective in inhibiting corrosion.
In European practice, its use in the absence of additional zinc and epoxy protective coatings is discouraged where natural soil resistivity is below 750 ohm/cm.
In Europe and Australia, ductile iron pipe is typically manufactured with a zinc coating overlaid by either a bituminous, polymeric, or epoxy finishing layer.
No current AWWA standards are available for bonded coatings (zinc, coal tar epoxy, tape-wrap systems as seen on steel pipe) for ductile iron pipe, DIPRA does not endorse bonded coatings, and AWWA M41 generally views them unfavourably, recommending they be used only in conjunction with cathodic protection.
They came in use in the early 1990s, replacing coatings based on dangerous and environmental harmful solvents, such as benzenes, toluenes, hexanes and other volatile organic compounds.
The association provides research on and promotes the use of ductile iron piping in utility projects (water and sewer), focusing on its strength, recyclability and life cycle cost compared with alternative products such as PVC.
[25] Following the financial crisis of 2008, the pipe industry as a whole, experienced a decrease in sales in the US due to municipalities deferring replacement of water mains and reduction in new home construction.
This study found improved environmental performance for ductile iron pipe in terms of energy consumed and emissions produced during manufacture due to its longer life span.
They found that at diameters of ≤ 24 in, ductile iron pipe had the highest "global warming potential" based on emissions from manufacturing, transportation and installation.
At larger diameters, ≥ 30 in, ductile iron pipe had a lower "global warming potential", while PVC had the highest.
